PSI - Issue 57

SEVEDE Théo et al. / Procedia Structural Integrity 57 (2024) 335–342 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

337 3

2. Experimental protocol

2.1. Ultrasonic fatigue machine

VHCF fatigue testing requires the use of a specific machine capable of achievinga high loading frequency [5]. In the framework of this study, the specimen is subjected to tension-compression loading at a frequency of 20 kHz with a load ratio R=-1. The working principle of this machine is based on the generation of a longitudinal elastic displacement wave using a piezo-electric generator at a resonance frequency of 20kHz. The displacement amplitude is then amplified before being applied on the sample. The free vibration wave propagates through the sample and then apply the displacement amplitude (Figure 1).

Figure 1:VHCF experimental set up: (a) ultrasonic fatigue machine and (b) its representative diagram

All the parts in the chain (converter, amplifier and the sample) are designed to have a resonance frequency equal to the excitation frequency of the piezo-electric generator, i.e., 20 kHz. In this way, the displacement amplitude is maximum at the sample extremities, while the center is a displacement node (zero-amplitude). This generates a heterogeneous stress field along the sample length, which is zero at the extremities and maximum in the center as shown in Figure 2.

Figure 2: self-heating testing protocol (a) ultrasonic sample geometry and (b) mechanical loading and temperature elevation profile versus time for one block at a given point of the specimen